I ran with a 'spare' M1 Pro MacBook Pro for a couple of years (my main Mac is also an M1 Pro) but if it's to be treated as a 'hot spare' you really need to use it to ensure it works by using it. After all, by purchasing another one to 'sit on the shelf' you are running the warranty down.
In the end, I had AppleCare+ on my main Mac (still not claimed on it) and ended up selling the 'spare' after it only got light use around the house (by using it I also made sure it was working just fine). For me, as times goes on you'll have access to more modern Macs year by year. If an off the shelf model (rather than a BTO) is ok as an emergency purchase then I'd advise putting the money in a bank until the day you need it and ensure you have a good backup regime for important data. And on top of that you'll get macOS updates for longer too.
In the long term I am concerned about macOS support ending at the same time so I'm happy to buy another Mac in the M5 or M6 generation to ensure I have a Mac that's getting updates going into the future.
Think about this. If you bought an M4 Mini now, by October that money could have bought you an M5 Mini. And in a couple of years time maybe an M6 mini? All this time the money is sitting in the bank earning interest - ok we don't know where prices will be by then but waiting a couple of years gets you a better Mac in case you needed it later. And in 5-6 years time you might be cursing as Apple announces the end of primary M4 support once macOS 32 comes out.
TLDR - Keeping an M4 Mini in the box 'just in case' is wasteful.
@kathryne_B - take your Mini out of the box and get it booted up to make sure it works - I don't think you're going to get bonus points for keeping an M4 mini brand new in the box for years. Once you think it works ok then put it back into storage.